Whenever I try to make them woohoo, the interaction to talk to their partner about woohoo (the "foreplay" thing) almost happens, but as soon as they get close, it cancels itself. I should mention that ErrorTrap doesn't make a script error when this happens, so that isn't very helpful.
GIF of the glitch:

While it's okay to bump an unanswered question after a few days have gone by, please do not post the same issue in multiple places on the message boards (duplicate deleted). Thanks.

Can ANY teen sims woohoo or is the issue just with these two?

We can see that shower woohoo is being targeted there. I assume you have Generations in play, yes?

And how are you directing these two sims to woohoo? Is it through Romantic interactions up through Sim A thinks Sim B is being flirty, alluring, all the way to irresistible and then the Romantic ones for Woohoo (plus maybe Risky and Try for Baby) show up or are you clicking directly on the shower? Can they woohoo in a bed or some other location? Can they they do this the other way by cuddling on a bed and work up through kissing, making out, and then finally woohoo without going through the Romantic interactions?

Also, the placement of the chosen woohoo object (the shower) might be at fault as it's in a tight corner. It's possible the two sims cannot route to it to complete the action.

While it's okay to bump an unanswered question after a few days have gone by, please do not post the same issue in multiple places on the message boards (duplicate deleted). Thanks.

Can ANY teen sims woohoo or is the issue just with these two?

We can see that shower woohoo is being targeted there. I assume you have Generations in play, yes?

And how are you directing these two sims to woohoo? Is it through Romantic interactions up through Sim A thinks Sim B is being flirty, alluring, all the way to irresistible and then the Romantic ones for Woohoo (plus maybe Risky and Try for Baby) show up or are you clicking directly on the shower? Can they woohoo in a bed or some other location? Can they they do this the other way by cuddling on a bed and work up through kissing, making out, and then finally woohoo without going through the Romantic interactions?

Also, the placement of the chosen woohoo object (the shower) might be at fault as it's in a tight corner. It's possible the two sims cannot route to it to complete the action.

Ok, sorry about the duplicate.

I'm not sure whether or not it's just this couple.

Yes, I have Generations.

I tried Romantic interactions, I tried direct object interactions, I tried different locations (bed, shower, treehouse, etc.). The same outcome happened, the one in the gif.

I haven't tried the cuddling method though, I might try that the next time the couple meets up.

At least that tells us that the two sims are attracted enough to each other to want to woohoo together in at least some circumstances. Yes, that cuddle sequence is a bit odd until one gets used to it, but it's provided for by the game itself as an alternative to the usual chain of Romantic interactions method.

Teen woohoo is blocked by the game core. Of course we know that Woohooer has the capacity to unlock it for us, but it just might be that the game itself is fighting back on the mod a bit more strongly on this couple than otherwise. Or, I did neglect to mention this, the blockage could also have been situational if either of them had low motives (hunger, bladder, etc.) or was experiencing a teen mood swing at the time, things like that, or had a trait that made them less interested in such behavior. But I take you had already ruled much of that out already.